FIG. 2 is a partly sectional perspective view showing the structure of a conventional aluminum electrolytic capacitor.
A capacitor element 29 in FIG. 2 is prepared by rolling an anode foil 21 and a cathode foil 22 through a separator 23, wherein the anode foil 21 is obtained by etching a surface of an aluminum foil to increase its effective surface area and subjecting the surface to a chemical conversion treatment to form a dielectric oxide film thereon, and the cathode foil 22 is obtained by etching a surface of an aluminum foil. An anode lead 25 and a cathode lead 26 are connected, respectively, to the anode foil 21 and the cathode foil 22 of the capacitor element 29.
Then, after the capacitor element 29 is impregnated with driving electrolyte 24 and inserted into a metal case 28, such as an aluminum case, the metal case 28 is sealed with a sealing member 27 made of rubber or the like to provide the aluminum electrolytic capacitor as shown in FIG. 2. The above sealing can be achieved by inserting the sealing member 27 into the metal case 28 and then hermetically closing the metal case 28 through curling and drawing.
In connection with the recent need for downsizing and higher reliability of electronic devices, it is required to provide a downsized aluminum electrolytic capacitor, and a surface-mount type aluminum electrolytic capacitor to be mounted directly on a surface of a printed board by reflow soldering is being developed.
One example of a sectional view of the surface-mount type aluminum electrolytic capacitor is shown in FIG. 3 (see Japanese Patent Laid Open Publication No. 09-275045 (Patent Publication 1), and Japanese Patent Publication No. 04-019695 (Patent Publication 2)).
In FIG. 3, the reference numeral 31 indicates a capacitor element; 32 indicates a metal case receiving therein the capacitor element 31 impregnated with driving electrolyte (not shown); 33 indicates an elastic sealing member for sealing an open end of the metal case 32; 34 and 35 indicate, respectively, an anode lead and a cathode lead which are led from the capacitor element 31; and 36 indicates an insulative seat plate. The insulative seat plate 36 is provided as a means to protect the capacitor element 31 from heat during reflow soldering.
A process of producing a surface-mount type aluminum electrolytic capacitor, which is disclosed in the above Patent Publications 1 and 2, is required to include an operation for disposing the insulative seat plate 36 in such a manner as to come into contact with a curlingly-sealed open edge of the metal case 32, and then bending the anode and cathode leads 34, 35 each having a flattened top end in such a manner that they are fitted in a concave portion formed in an outer surface of the insulative seat plate 36. This assembling operation leading to increase in process time causes problems about considerable complication in production process and increase in production costs.
Moreover, the surface-mount type aluminum electrolytic capacitor additionally including the insulative seat plate 36 as compared with a conventional discrete-type aluminum electrolytic capacitor involves another problem about increase in overall dimension due to the thickness of the insulative seat plate 36.
In order to solve the above problems, Japanese Patent Laid Open Publication No. 04-012514 (Patent Publication 3) proposes an aluminum electrolytic capacitor designed such that anode and cathode leads are bent along an outer surface of a sealing member without providing any insulative seat plate.
However, the surface-mount type aluminum electrolytic capacitor disclosed in the above Patent Publication 3 cannot assure an adequate heat resistance due to nonuse of an insulative seat plate, and involves a problem that, when the aluminum electrolytic capacitor is surface-mounted on a board by reflow soldering, a sealing member is liable to be deformed to cause a problem about the occurrence of a mounting defect.
Particularly as to mounting means for electronic devices, in late years, the use of a solder containing no Pb (Pb-free solder) has been promoted as one of measures to protect the global environment. The Pb-free solder has a melting point (about 240 to 270° C.) greater than that (about 220 to 240° C.) of a conventional solder, and the reflow soldering using the Pb-free solder is performed at a higher reflow temperature than a conventional reflow temperature. Accordingly, the level of heat resistance required for recent aluminum electrolytic capacitors becomes higher than ever before, and thereby the above aluminum electrolytic capacitor using no insulative seat plate has difficulties in practical use due to frequent occurrence of the deformation of the sealing member and the resulting mounting defect.
As a conventionally known material of a sealing member, for example, a rubber material such as peroxide-cured, resin-cured, sulfur-cured or quinoid-cured butyl rubber, and peroxide-cured or sulfur-cured ethylene-propylene rubber was described in the Patent Publication 1.
However, in the rubber material disclosed in the Patent Publication 1, a reinforcing filler has to be added thereto in a larger amount to assure a desired heat resistance (an elastic modulus at high temperature) capable of withstanding the reflow soldering using the Pd-free solder. In case where the reinforcing filler is added in a larger amount, the content of rubber component will be relatively reduced and maintaining of highly air-tightness sealing property in the capacitor element will be difficult due to insufficient elasticity as a material of the sealing member. Specifically, the operation for hermetically sealing the capacitor element in the metal case includes drawing and curling. If the sealing member has a low elasticity, the drawing and curling can hardly be performed to keep the sealing member in close contact with the metal case so as to obtain an adequate sealing performance, and are likely to cause cracks in the sealing member.
With a view to maintaining the elasticity of the sealing member in cases where a reinforcing filler is added to the sealing member in a larger amount to provide enhanced heat resistance in the rubber component thereof, the inventers also made researches on a mean of lowering the crosslinking degree of the rubber component.
However, if the crosslinking degree of the aforementioned conventional butyl rubber is lowered, its hardness will also be lowered. Thus, when an aluminum electrolytic capacitor having no insulative seat plate and using the sealing member made of the butyl rubber with a low crosslinking degree is surface-mounted on a board by the reflow soldering using the Pb-free solder, the sealing member is easily deformed due to its low hardness, which is likely to cause an undesirable phenomenon, such as leakage of the driving electrolyte, and/or a mounting defect. For this reason, it is difficult to put the above mean to practical use.
It is therefore an object of the present invention to provide a surface-mount type aluminum electrolytic capacitor to be mounted on a surface of a board by reflow soldering, capable of obtaining an adequate sealing performance without the occurrence of cracks and other defect in a sealing member during drawing and curling in a production process for the capacitor, and suppressing the occurrence of a mounting defect and other defect due to deformation in the sealing member even at a reflow temperature of a Pb-free solder without providing any insulative seat plate.